Commerical mixtures (Aroclors) of polychlorinated biphenyls (PCBs) are more toxic to rhesus monkeys (Macaca mulatta) than to rats. Furthermore, the 3, 4, 3',4'-tetrachlorobiphenyl is much more toxic than Aroclor 1242 for rhesus monkeys. We are assessing the oral toxicity of two tetrachlorobiphenyls and two hexachlorobiphenyls for rhesus monkeys by use of criteria of clinical appearance and squamous gland metaplasia. One member of each pair of equally chlorinated compounds has chlorine substitutions in the ortho position, and the other has only meta and para substitutions. Test feedings will stop when toxicity is apparent, and recovery on normal diet will be followed for a year. Samples of body fat, blood, and excreta are periodically analyzed for the levels of PCBs and the appearance of metabolites during both experimental feeding and recovery. Metabolites found in monkeys made sick by a given PCB will be definitively identified if possible, and their toxicity will be evaluated whenever enough compound can be synthesized for feeding experiments.